[Cashtab] Use textarea instead of input for mnemonic imports

Summary: Bad UX to use input here as the mnemonics are not fully visible. Use a textarea so user can see full seed on import. Easier to edit error in earlier words.

Test Plan:
npm test, screenshots

before

after

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18805